This proposal addresses the important issue of hormone therapy failures among post-menopausal women with estrogen receptor- positive ER(+) breast cancer with or without progesterone receptors (PR). Clinical correlation studies have shown that the current methods of sub-classification of ER(+) tumors based on the presence/absence of proteins which are markers of estrogen action (ex. PR) are inadequate for precise differentiation of truly hormone-responsive from hormone-resistant ER(+) tumors. The level of prediction error (30% ER+/PR+ failures and 25-30% ER+/PR- responders to hormone therapy) translates to significant proportions because 75% of 100,000 newly diagnosed cases of breast cancer will be ER(+) and 50-60% of these will be PR(+). A precise sub-classification of primary biopsies of ER(+) tumors will be valuable for early selection of potential hormone therapy failures when the tumor burden is small so that alternative therapies can be administered to benefit the patient. With the help of polyclonal antibodies (Pab) to ER, in-vitro hormone incubation procedure and immunohistochemistry (IF) developed in our lab we have identified two varieties of ER in human cancers which, although capable of binding estrogens with high affinity, are defective in their nuclear binding function. Preliminary clinical correlation data have also revealed a preponderance of these defective ER among those breast cancers which are hormone resistant and of non-defective ER among those which are responsive to hormone therapy. Based on these results and of previous investigations emphasizing a correlation between defective ER and hormone- independent growth WE PROPOSE THAT A SUB-CLASSIFICATION OF ER(+) TUMORS BASED ON NUCLEAR BINDING CHARACTERISTICS MAY DISCERN TRULY HORMONE-DEPENDENT TUMORS FROM RESISTANT ONES. Our SPECIFIC GOALS , therefore, are: (1) To develop novel monoclonal antibodies (Mabs) to identify and discriminate among the defective/non-defective ER based on their epitope differences; (2) To immunotype with Pabs and IF, the ER in established ER(+) human breast cancer cell lines with known hormone response data; (3) To attempt establishment of primary cultures from malignant fluids of patients with ER(+) tumors who failed hormone therapy and to immunotype ER with Pabs; (4) To immunotype sequential biopsies of ER(+) tumors (primary vs recurrent) to find out if recurrences become enriched in cells with defective ER; to increase patient population for clinical response to hormone therapy.